External photoemitters (or photocathodes) are known in the art as a means to detect optical radiation. These types of detectors can be used, by way of example, in image intensifiers for light amplification in the visible part of the electromagnetic spectrum. Practical applications can include their use in night vision devices such as binoculars, scopes and goggles, infrared viewers, streak camera tubes for high speed photography, and the like.
Some applications can use photocathodes with multichannel plates (MCPs) to detect low levels of electromagnetic radiation from a photocathode surface. MPCs can accelerate electrons but coming from the photocathode, provide gain.
Recent advances have included high efficiency III-V negative electron affinity photocathodes. These devices can be especially sensitive to their surroundings and must be developed within an ultra-high vacuum to achieve maximum sensitivity. These devices are also limited to detecting wavelengths no longer than about 2 μm due to the formation of a heterojunction barrier at the surface that is formed. An example of this type of device has worked for field-assisted photoemission using p-InGaAs.
While there have been significant advances in the art, further advances are possible and desired. It would be desirable to have an external photoemissive detection and processing where wavelengths beyond 2 μm are possible. For example, it would be desirable to detect radiation from 1-20 microns.
Further, atmospheric windows are ranges of electromagnetic radiation wavelengths that are allowed to easily pass through the atmosphere to Earth's surface. This is in contrast to EM wavelengths that are absorbed by atmospheric gases such as water vapor, carbon dioxide and ozone, known as absorption bands. It would be desirable to efficiently detect radiation within multiple atmospheric windows, such as those at 1-2 μm, 3-5 μm and 8-14 μm. This could be especially valuable if this type of detection could be accomplished with a single film of material.
Additional improvements could be a simplification of manufacturing, such as the ability to produce these types of detectors under modest vacuum conditions and be capable of exposure to atmospheric conditions for extended periods without essentially compromising its operation. Such a device could be readily manufactured with an added benefit of reduced need for toxic materials.